This invention relates to a system for controlling a generator, more particularly to a system for controlling the field excitation of a synchronous generator by means of a digital electronic computer.
Due to a rapid increase in the population of the world and improvement in the living standard, the energy comsumption of the world is increasing year after year, and the total energy consumption during the interval of from 1970 to 2050 is estimated to be about 100 Q (1Q=3.times.10.sup.14 KHW). The total amount of available fossil fuels including coal, petroleum and natural gas is estimated to be about 100Q. In other words, at about the middle of the 21th century, the fossil fuel will be exhausted. In addition, combustion of fossil fuels results in pollution of the atmosphere which greatly affects the weather of the earth as well as the living environment of life. For this reason, development of new energy sources is necessary, especially in countries deficient in the natural resource of fossil fuels.
One of the energy resources other than fossil fuels is solar energy. The technique of utilizing solar energy is called sun shine project. However, solar energy now actually used is only a fraction of the total energy available, and many problems still remain to be solved in the future. Another energy source is hydroelectric power. However, the amount of hydroelectric power available per year is only about 0.05 Q which is only a fraction of the total energy demand. Other resources include wind power, tidal power, the heat of the earth, etc., but the energy generated by these resources is very small.
The only other abundant energy resource is nuclear energy which can be released by fusion reactions and fission reactions. The latter reactions are now utilized in nuclear power plants. However, the amount of uranium and thorium ores that can be used is estimated to be only about 100 Q. Moreover, there are many problems such as radioactive discards, risk of run out as well as limits on the site of the plant.
As is well known in the art the fusion reaction involves a D-D reaction and a D-T reaction, where D represents deuteron and T tritium. Deuterons used in the D-D reaction are present abundantly in sea water and estimated to amount to 10.sup.10 Q. The fusion reaction is a reaction in which two light weight atomic nuclei are caused to collide with each other at a speed higher than a critical speed so as to cause an exothermic reaction thereby forming a slightly heavier nuclei, for example helium. In nature, such reactions are found in lightening, aurora, ionized layers and the sun. Accordingly, in order to utilize the fusion reactions as a source of energy it is necessary to establish a state (a high temperature plasma) resembling the state existing in the central portion of the sun. In the case of the sun, the high temperature plasma state is confined in the central portion by gravity, that is a huge mass. On the earth, however, as it is impossible to use such a huge mass, a magnetic field is used.
The basic construction of a fusion reactor of laboratory scale is shown in FIG. 1. As shown, a primary winding 10.sub.2 and an annular tube 11 containing inonizable gas and acting as a secondary winding are wound on the center leg 10.sub.1 of the core of a transformer 10. When current Ic flowing through the primary winding 10.sub.2 is varied rapidly a plasma current Ip flows through the ionized gas. The fusion reaction is effected by the Joule heat generated by the plasma curent Ip. Since such fusion reaction terminates in an extremely short time, it is necessary to seal new ionized gas in the discharge tube 11 for passing plasma current. In a present day fusion experiment, it is necessary to repeat such cycle of operation at an interval of several minutes.
As the plasma comes into contact with the wall surface of the annular discharge tube 11, the plasma is cooled so that the plasma current diminishes rapidly. Accordingly, it is necessary to confine the plasma inside of the tube or at the central portion thereof so as to prevent the plasma current from coming into contact with the wall surface. To this end, a toroidal field coil 12 is wound about the discharge tube 11.
If the current I.sub.T flowing through the field coil takes the form of a regular rectangular waveform as shown in FIG. 2a, the amplitude of the field is constant during an interval T between the build-up and build-down of the current. This means that the pimary current Ic or the plasma ignition may be passed at any time during the interval T which is advantageous in that not only is the experiment greatly facilitated but also the power loss of the field coil 12 is zero until the current I.sub.T bulds up to a prescribed value.
Actually, however, it is impossible to make rectangular the field coil current I.sub.T as shown in FIG. 2 due to the time constant of the source supplying the current I.sub.T and the time constant of the coil 12 itself. More particularly, the capacity of the source must be large in order to confine the high temperature plasma current in the tube by a magnetic field but it not necessary to continuously pass current. Thus it is necessary to pass current for an extremely short duration of an order of one second at an interval of several seconds. Accordingly, the utilization factor of the source is very low and it is not permissible to periodically supply such large power directly from distribution lines. A turbine generator set may be installed instead of relying upon existing distribution lines but turbine generator sets introduce problems of noise, polution of the air by smoke and large installation cost. Thus, such an independent power source is not suitable for research purpose. As described above, since it is necessary to pass a large current through the field coil 12 for only a short duration, a flywheel type motor generator set is most suitable for this purpose.
In a flywheel type motor generator set the flywheel is rotated by the motor to store energy in the flywheel and the stored energy is converted into electric energy by applying an excitation to the generator at required times. Such set is generally constructed as shown in FIG. 3, and comprises a driving motor 13, a flywheel 14 and a synchronous generator 15 which are directly coupled together. In one example, the generator has a capacity of 100 MW, the flywheel 14 has a flywheel effect of 33.5 ton m.sup.2 and the motor 13 has a capacity of 2600 K.W. In this manner, the power rating of the motor may be only about 1/40 of the power source from which the field coil is energized. For this reason, flywheel type motor generator sets are used exclusively in most fusion laboratories.
Generally, the time constant, or the so-called open circuit time constant Tdc of the armature winding of the generator of a motor generator set is designed to be about 5-6 seconds. Accordingly, even when an excitation current is passed in the same manner as in ordinary generators, the output voltage would reach the prescribed value only after 5 to 6 seconds. Further, since the field coil 12 of a fusion laboratory has a time constant of about 0.8-1 second, it takes a substantial time until the field coil current I.sub.T builds up to the prescribed value after application of the field excitation.
In order to decrease the field coil loss and to establish a magnetic field which is maintained at a stable value for a given duration (for example, one second) it is advantageous to make the field coil current have a wave form in the shape of a frustum as shown in FIG. 4. However, it is difficult to produce a field coil current I.sub.T having such waveform by an the analogue feedback control system shown in FIG. 5 which has been used in many cases. In the control system shown in FIG. 5, the field winding I.sub.F of a generator 15 driven by motor 13 is supplied with field current I.sub.F from an excitation source 17 via a thyristor 17.sub.1. The gate signal applied to the gate electrode of thyristor 17.sub.1 is derived from the generator though a rectifier 16, a DC transformer 19 for deriving a feedback signal F, a comparator C which compares feedback signal F with a reference signal R, and an amplifier 18. FIG. 6 is a block diagram showing an equivalent control system of the circuit shown in FIG. 5. This analogue feedback control system is characterized in that it operates stably regardless of the variation in the parameters of the control system (for example, variations in the field resistance caused by temperature rise) and that it is possible to eliminate any offset error by incorporating an integrating element into the control system.
In order to produce a current having a waveform as shown in FIG. 4 by this control system it is necessary to greatly increase the response speed of the control system. Although the system quickly responds to a given reference signal, overshooting and undershooting and hence hunting are inevitable because the feedback control system functions to follow up a deviation. Accordingly, the higher the response speed, the larger the hunting.
The solid line shown in FIG. 7 represents the current waveform where the response speed of the control system is increased. In order to decrease the overshoot it is necessary to provide an overdamping by decreasing the gain of the system and to increase the feedback as shown by the dot and dash lines shown in FIG. 7.
In any case, these curves depart from the desired waveform shown by the dotted lines. As described above, in order to maintain the plasma confining field for more than one second it is necessary to maintain the flat portion of the current at a prescribed value for about one second. However, with a conventional analogue feedback control system, due to its time constant, the current begins to decay immediately after it has reached the prescribed value. For this reason, the prior art control system is not applicable to an electric source for fusion reaction laboratory equipments.
As a system for obviating these difficulties of the prior control systems a so-called optimum control system has been developed wherein control is performed in a minimum time while satisfying various restricting conditions of the component elements of the system. One example of the optimum control system involves a dynamic programming proposed by Bellman. However, this method requires that a large memory device and an electronic computer be used for solving cubic or more complicated equations. Thus, to solve the problems under restricting conditions, the computer is required to perform operations consuming many hours and to use a memory device having an extremely large capacity, for example several tens thousand words. In this manner, it is impractical to apply such dynamic programming to a control system because of its high price and time consuming operation.